My Webiste currently have 289 pages indexed by google , I am thinking to double up the page to 600 pages . But my question is , will more pages indexed by google boost your search ranking ?
You need to think of your website in the following manner - every page of your site is a door to your site - therefore every page indexed is another opportunity for your site to be found. That's part of the reason why Google loves sites like cnn.com - they're constantly creating new pages, and have many entry points to their site.
The more the pages in search engine indexes, the more the chances of your website being found. As rightly pointed out by RankSurge, indexed pages act as a door way to your website and hence is potentially important to you SEO initiatives.
The short answer is yes. More content will mean more long tail search engine hits. More content can also help your main keywords if you focus it on your main site theme. What you want to do is try to focus every page on a sub topic of your main site. Get more pages on the sub topics and it will help Google recognize the main topic of your site. Then try to get links back to your site for the main topic and some of the more important sub topics. Over time you'll see all the extra pages rising in the SERPs and your main keywords too.
To my opinion having more content is always good. but the number of searches depends on quality of content. having many pages in google index does not increase searches but the pages must be search able also. Once new pages start coming in searching you will get more search. if you site pages are on smiler topic then you will get higher search as site will become niche site on that topic. just increasing pages on any thing will not help you much.